1. Field of the Invention
This invention relates to an underwater wide angle lens.
2. Related Background Art
When photographing which requires a wide angle of field of about 180 is to be effected under water, a land camera and a fish-eye lens have heretofore been used and photographing has been effected with these contained in a semispherical underwater housing.
However, when a camera having a land fish-eye lens mounted thereon is used under water while being contained in an underwater housing, aberrations including astigmatism have been created in the inner and outer boundary surfaces of the semispherical underwater housing and therefore, the imaging performance of the fish-eye lens has been Greatly reduced.
Also, when underwater photographing is effected with a fish-eye lens which is similar in lens construction to a land lens being mounted on a camera within an underwater housing, the medium on the object side turns from air into water and the refractive index of the object side becomes Greater than that of the image side and therefore, the focal length of the lens increases in conformity with the ratio (about 1.33 times) of refractive index of water to air and as compared with the specification during use on land, a reduction in the photographing angle of field under water occurs and therefore, wide image photographing has been impossible and the feature of the fish-eye lens which is a wide angle of field has been spoiled.
On the other hand, in order to eliminate various problems including not only reductions in various optical performances caused by a land lens being used under water by the use of an underwater housing, but also a reduction in operability caused by a camera body being hermetically sealed in the underwater housing, a single-lens reflex camera exclusively for underwater use which is endowed with a waterproof function has been developed and along therewith, there is a demand for an underwater wide angle lens having a waterproof function which can be mounted on such a single-lens reflex camera exclusively for underwater use.